1. Field of the Invention
The present invention relates to methods for producing semiconductor laser devices, in particular, high-output semiconductor laser devices which are used for optical disks, the generation of second harmonics, the oscillation of solid-state lasers, laser beam printers, the oscillation of optical fiber amplifiers, optical communications, laser beam processing, and laser treatments.
2. Description of the Related Art
In recent years, semiconductor laser devices having such advantages as small size, high efficiency, low cost, etc., have been put to practical uses, and thereby applied to general industrial and consumer appliances in which a laser beam source had been difficult to use in the past. The semiconductor laser devices having these advantages are expected to be used for the formation of high-speed optical disks, the effective generation of second harmonics and solid-state laser beams, the formation of high-speed laser printers, the extension of a relay distance and the formation of high-speed transmission in optical communication systems using optical fiber amplifiers, the formation of remarkably compact laser beam machines and laser treatment machines by making it possible to operate the semiconductor laser devices having these advantages at a high output power level.
However, the semiconductor laser devices as mentioned above have a problem such that the light-emitting end facets are damaged due to high optical density when the devices are operated at a high output power level. In order to overcome this problem, the following methods are known to be effective:
(1) reducing the optical density of the light-emitting end facets by extending the stripe width of a laser oscillation waveguide, PA1 (2) reducing the optical density of the light-emitting end facets by extending the light in the vertical direction toward the stripe width direction of the laser oscillation waveguide, PA1 (3) forming a current non-injection region in the vicinity of the light-emitting end facets, and PA1 (4) forming semiconductor layers which are lattice-matched with an internal semiconductor crystal and have a wider band gap than an active layer in the vicinity of the light-emitting end facets, whereby an interface state generating at the light-emitting end facets is removed to make them light non-absorption layers. PA1 forming the window layers made of a semiconductor on either one of a top surface of the layered structure or a reverse surface of the substrate and on the light-emitting end facets of the layered structure; PA1 forming a reflection film and/or an anti-reflection film on the light-emitting end facets; PA1 removing the window layer formed on either one of the top surface of the layered structure or the reverse surface of the substrate by using an etchant which hardly etches the reflection film or the anti-reflection film; and PA1 forming electrodes on the surface from which the window layer is removed by etching and on the other surface. PA1 forming window layers on light-emitting end facets of the bars; PA1 inserting the bars with the window layers into an apparatus, the apparatus having openings for forming electrodes on top surfaces of the bars and on reverse surfaces thereof and a supporting portion for preventing a positional shift between the bars and the openings; and PA1 forming the electrodes on the top surfaces and the reverse surfaces of the bars under the condition that the bars are in the apparatus; and PA1 cutting the bars on which the electrodes are formed into the chips.
A semiconductor laser device with a high output laser beam may be prepared by combining these methods.
One example of the methods (4) comprises forming the light-emitting end facets by a cleavage or etching technique, and then forming very thin light non-adsorption layers (hereinafter, referred to as window layers) on the light-emitting end facets. According to this method, when a waveguide light inside the device is reflected on the light-emitting end facet and bound to a waveguide again, there is little loss of light caused by the diffusion on the window layer. Thus, there is an advantage that the laser beam has hardly deteriorated oscillation efficiency compared to the case where no window layer is formed.
The window layers can be formed by the molecular beam epitaxy (MBE) method, or chemical vapor deposition methods such as the metal organic chemical vapor deposition (MOCVD) method.
However, because of the high degree of vacuum and the lack of the crystal growth beyond each facet in the MBE method, it is necessary to form separately the window layer at the front and back facets. On the other hand, because of the low degree of vacuum and the presence of the crystal growth beyond each facet in the MOCVD method, it is possible to form the window layer at the front and back facets, simultaneously.
An example of growing window layers on light-emitting end facets by the above-mentioned MOCVD method is shown in FIG. 12a. First, a wafer with an internal structure (layered structure) including an active region is cleaved to form a bar 2 in which the front and back end facets are exposed. The bar 2 is inserted into a slot in a holder 905 so that a reverse surface (substrate side) is faced down, and placed in an MOCVD device. Then, as shown in FIG. 12b, AlGaAs high resistance layers 930A, 930B, and 930C are formed on the front end facet, the back end facet, and a top surface of the layered structure, respectively. At this time, the reverse surface is in contact with the holder 905, so that the AlGaAs high resistance layer is not formed on the reverse surface. The AlGaAs high resistance layers formed on the front and back end facets 930A and 930B function as window layers.
However, the AlGaAs high resistance layer 930C (unwanted growth layer) formed on the top surface of the layered structure increases the resistance of the semiconductor laser device. In particular, when the semiconductor laser device operates at a high output power level, problems arise such as efficiency deterioration and short lifetime because of the increase in resistance of the device. In the case where the bar 2 is inserted into the holder 905 so that the top surface of the layered structure is faced down, an AlGaAs high resistance layer (unwanted growth layer) is formed on the reverse surface, which still increases the resistance of the semiconductor laser device.
In order to prevent the formation of the above-mentioned unwanted growth layer, the top surface of the layered structure or the reverse surface are covered. As shown in FIG. 13a, the bar 2 and a cover 903 are inserted into the slot of the holder 905 which is placed in the MOCVD device. Stock gas is not provided to the underneath of the cover 903, and unwanted growth layer is not formed on the top surface of the layered structure. However, actually, in order to readily insert the bar 2 into the slot, the width of the slot is made relatively larger than that of the bar 2. Because of this, even though the width of the bar 2 and that of the cover 903 are identical, a shift in their relative positions is caused as shown in FIG. 13b. Due to this shift, the AlGaAs layer 930C (unwanted growth layer) is formed on a portion of the internal structure at one side of the end facets. At the other side of the end facets, the cover 903 prevents the flow of the stock gas because of the positional shift between the bar 2 and the cover 903, so that the window layer 930B does not have a uniform thickness. In an extreme case, the window layer 930B may not be formed at all at this side.
To solve this problem, an etching technique has been proposed to remove the unwanted growth layer. However, in this method, because the composition of the window layers is the same as that of the unwanted growth layer, it is necessary to form protective layers against an etchant on the window layers to prevent the window layers from being etched when the unwanted growth layer is etched. After the unwanted growth layer is removed, an additional step of removing the protective layers is required, which increases the number of the steps, and also provides a very complicated process.
On the other hand, in the case where the window layers are grown on the light-emitting end facets, the window layers are generally formed before the formation of electrodes so as to prevent a crystal growing device from being contaminated. In this case, the wafer of the facet is cleaved to form a number of bars so as to expose the light-emitting end facets before the formation of the window layers. Because of this, it is necessary to form the electrodes on the respective bars whose number is several tens to several hundreds times as many as one wafer. Moreover, each bar has a small width, for example, of 400 .mu.m, so that the handling of each bar is complicated. Also, it is necessary to form the electrodes on the very limit of the small width of the bars, and the electrodes are not allowed to be formed toward the end facets thereof. Furthermore, the electrodes need to be formed on the top surface of the layered structure and the reverse surface, each at least once. In general, the electrode's formation should be conducted twice for each surface.
A procedure for forming the electrodes in the semiconductor laser device by forming the electrodes on the respective bars will be described below. As shown in FIG. 14a, the bars 2 are inserted into a holder 901 with grooves for supporting both end facets of the bars 2 so that each top surface of the layered structure is faced up. Then, a mask 902 having openings with a width smaller than that of the bars 2 are placed on the resulting holder 901 as shown in FIG. 14b to fix the holder 901 and the mask 902. Under this condition, the holder 901 is placed in a vacuum deposition machine so that the openings of the mask 902 are faced down (facing a deposition source), and then AuGe/Ni electrodes are formed on the top surfaces of the layered structure of the bars 2 by vapor deposition. After that, the mask 902 and the holder 901 are taken out of the vacuum deposition machine and they are separated from each other. Then the bars 2 are inverted and inserted into the holder 901. After fixing the mask 902, the resulting holder 901 is placed in the vacuum deposition machine so that the openings of the mask 902 are faced down, and AuZn electrodes are formed on the reverse surfaces of the bars 2 by vacuum deposition. After that, the bars 2 are taken out of the holder 901 and heated in a heat treatment furnace at 450.degree. C. for 10 minutes. The bars 2 are inserted into the holder 901 so that each top surface of the layered structure is faced up and the mask 902 is fixed. The resulting holder 901 is placed in the vacuum vapor deposition machine, and Mo/Au electrodes are formed on the AuGe/Ni electrodes by vacuum deposition. The bars 2 placed in the holder 901 are inverted, and then Al electrodes are formed on the AuZn electrodes by vacuum deposition. In this way, the electrodes are formed on the top surface of the layered structure and the reverse surfaces of the bars, other than the end facets thereof.
As described above, the formation of the electrodes on the bars is very complicated. Thus, even though high output semiconductor laser devices in which window layers are formed on cleaved faces (light-emitting end facets) have an advantage of high performance, they become very expensive, preventing their diffusion.